Through the use of cryptography, a message can only be read by its recipient. A key is used to encrypt the message. The owner of the key is the only person who can read the message received.
The encryption key must therefore be transmitted by the sender to the recipient of the encrypted message. Transmission is carried out such that only the recipient of the encrypted message receives this encryption key. Interception by a third party of the encryption key is detected by the sender or the recipient. Consequently, the encryption key or the elements of the key detected as having been intercepted are not used to encrypt the message.
The principle of transmitting encryption keys is used, for example, in quantum cryptography. It consists of using physical properties to guarantee the integrity of a received encryption key.
The encryption key consists of a bit sequence. Generally, a photon polarization state is associated with each bit. The light flow, encoded by polarization, is then attenuated. The probability of detecting two photons associated with the same bit is then negligible.
The sender can encode the encryption key on two nonorthogonal states (a given polarization state and a state at 45°). Concerning this subject, Bennett wrote the article “Quantum Cryptography using any two Nonorthogonal states” in Physics Review letters 68 in 1992. In reception, the detection states are chosen in a base with two states. These two detection states are orthogonal respectively to each state of the base used by the sender. During transmission, the transmission and detection states are chosen independently of each other.
If the states chosen by the transmitter and the receiver are orthogonal, the detection probability is zero. The measurement result is certain, there is no ambiguity. If they are not orthogonal, there are two possible measurement results since the probability of detecting the photon is 0.5. If the photon is detected, it is certain that the transmitter state is at 45° to the receiver state. There is no ambiguity. Irrespective of the polarization configuration, there is always a possibility of not detecting the photon. This non detection of the photon makes deducing the choice of transmitter polarization, using the receiver state, ambiguous.
This ambiguity concerning the polarization is used in quantum cryptography. A non recipient cannot reproduce the message since it is impossible to avoid losing information.
This type of quantum cryptography is known as “polarization ambiguity quantum cryptography” since it uses photon polarization states. A certain number of problems are to be faced. They concern the encoding of the encryption key on the polarization states of the photons in a light flow. During transmission, there is a problem of polarization distortion. For example, transmission by optical fibers requires complex systems which are difficult to implement and very expensive. For example,                either the use of polarization-maintained fibers, which are expensive and difficult to implement,        or the use of complex systems implementing, for example, Faraday rotators.        